Many phase change materials are referred to as GST because they are based on GexSbyTez material compositions. GST materials can also include additives, such as SiO2-GST, sometimes referred to below as additive GST.
Phase change memory cells have been designed so that there is a very small volume of phase change material in contact with one of the top and bottom electrodes. This serves to confine active region of the cell to a small volume of material, reducing the power needed for operation, and improving speed. One type of cell, sometimes referred to as a pore-type cell, is formed by making a small pore that forms an opening through an insulating layer to an electrode, and filling the pore with the phase change material. The size of the pore can be a critical dimension for the cell. For very small pores, it can be difficult to consistently and uniformly fill the pores in a memory array.
It has been found that it is possible to fill a very small hole, on the order of 17 nm×7.5 nm×30 nm deep, with some phase change materials using an atomic layer deposition (ALD) process or a chemical vapor deposition (CVD) process. However, it is not presently practical to fill such small holes with phase change materials using conventional plasma vapor deposition (PVD) processes.
One of the problems with the ALD process is that it is very difficult to control the phase change material compositions. Also, it is not possible to use ALD or CVD with many additives that are used with phase change materials.